Wireless communications protocols such as IEEE 802.11 (For any IEEE standards recited herein, see: http://standards.ieee.org/getieee802/index.html or contact the IEEE at IEEE, 445 Hoes Lane, PO Box 1331, Piscataway, N.J. 08855-1331, USA.) include management and control frames that support data transfer. The beacon frame, which is a type of management frame, enables stations to establish and maintain communications in an orderly fashion.
A typical beacon frame includes a common frame header and a cyclic redundancy checking (CRC) field. The header typically includes source and destination Medium Access Control (MAC) addresses as well as other information regarding the communications process. The destination address is set to the broadcast Medium Access Control (MAC) address. This forces all other stations on the applicable channel to receive and process each beacon frame. The CRC field provides error detection capability.
The beacon's frame body resides between the header and the CRC field and includes information about the beaconing device, such as the beaconing device's Media Access Control (MAC) address, the location of the beaconing device, the channels or sub-channels that are being used by the beaconing device or by the communication device associated with the beaconing device, the length of time that these channels will be used, the priority of the beaconing device, and the like. This information can be used for various purposes such as synchronization, checking signal strength of the beaconing device and determining proximity to the licensed device.
In infrastructure networks, access points periodically send beacons. In ad hoc networks, one or more of the peer stations assumes the responsibility for sending the beacon. After receiving a beacon frame, each station (also known as node, communication device, mobile node, or mesh node) waits for the beacon interval and then sends a beacon if no other station does so after a random time delay. This ensures that at least one station will send a beacon, and the random delay rotates the responsibility for sending beacons.
By increasing the beacon interval, the number of beacons and associated overhead is reduced, but that will likely delay the association and roaming process because stations scanning for available access points or neighboring stations may miss the beacons. Decreasing the beacon interval increases the rate of beacons. This will make the association and roaming process very responsive; however, the network will incur additional overhead and throughput will be reduced. In addition, stations using power save mode will need to consume more power because they'll need to awaken more often, which reduces power saving mode benefits.
There are no reservations for sending beacons, and they are commonly sent using the 802.11 carrier sense multiple access/collision avoidance (CSMA/CA) algorithm. If another station is sending a frame when the beacon is to be sent, then the sending station must wait. As a result, the actual time between beacons may be longer than the beacon interval. Stations, however, compensate for this inaccuracy by utilizing the timestamp found within the beacon.
When a beacon is found, the receiving station learns a great deal about that particular network. This enables a ranking of access points and other neighboring nodes based on the received signal strength of the beacon, along with capability information regarding the network. The station can then associate with the most preferable access point or other network device, for example.
After association, the station continues to scan for other beacons in case the signal from the currently-associated neighboring device become too weak to maintain communications. As the station receives beacons from the associated device, the station updates its local clock to maintain timing synchronization with the device and other stations. In addition, the station will abide by any other changes, such as data rate, that the frame body of the beacon indicates.
In a typical beaconing system, every device sends a beacon at a predetermined interval of time. This means there is an upper limit on the practical size/density of a network. Dense networks tend to have too much signaling overhead—including beacons. Adjusting the beacon rate based on density penalizes nodes that are in the center of the network, independently of actual airtime usage. Accordingly, there is a need for a method for adaptive beaconing.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.